Linear alpha-olefins are important materials used as comonomers, cleaners, lubricants, plasticizers, and the like, and are widely used industrially. Particularly, 1-hexene and 1-octene are used as comonomers for controlling the density of polyethylene when preparing linear low density polyethylene (LLDPE).
In the conventional preparation process of LLDPE, copolymerization with comonomers such as alpha-olefins, for example, 1-hexene and 1-octene, is conducted so that a branch may be formed in the polymer backbone together with ethylene to control the density.
Thus, in order to prepare LLDPE with high comonomer content, there is a problem in that the cost of comonomers accounts for a great part of the production cost. Various attempts have been made in order to overcome this problem.
Since alpha-olefins have different application fields or market fields according to the kind, technologies capable of selectively producing specific olefins is commercially important, and recently, many studies have progressed on chromium catalyst technology for preparing 1-hexene or 1-octene with high selectivity through selective ethylene oligomerization.
Existing commercial methods for preparing 1-hexene or 1-octene include the SHOP process of Shell Chemical, the Ziegler process of Chevron Philips, and the like, which may be used to produce a wide distribution of alpha-olefins ranging from C4-20.
As a trimerization catalyst of ethylene, a chromium-based catalyst using a ligand of general formula (R1)(R2)X—Y—X(R3)(R4) has been suggested. In the formula, X is phosphorus, arsenic, or antimony, Y is a linking group such as —N(R5)-, and at least one of R1, R2, R3, and R4 has a polar or electron accepting substituent.
A representative example of such ligands is (o-ethylphenyl)2PN(Me)P(o-ethylphenyl)2, which is a compound wherein at least one of R1, R2, R3, and R4 does not have a polar substituent (Chem. Commun., 2002, 858).
However, there is continued demand for development of ligands that have continuously continued oligomerization activity and high selectivity when preparing 1-octene or 1-hexene.